Electrical measuring instrument.



E. THOMSON.

ELECTRICAL MEASURING INSTRUMENT.

APPLICATION FILED JULY so, 1908.

1,U22,5 l '7. Patented Apr. 9, 1912.

2 SHBETSSHEET l.

1 i V i 1 i8 8 9 I ,4 1 u Fl 2 19 l4 s I r V Witnesses: I nventorz L M Elihu Thornspn. kw @ZW BY MMM E. THOMSON.

ELECTRICAL MEASURING INSTRUMENT.

APPLICATION FILED JULY so, 1908.

1,0212,517., Patented A rys, 1912.

2 snnms nnm 2.

Witnesses: Inventor; Elihu Thomson,

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ELIE-EU THOMSGN, 0F SWAIVIPSCOTT, MASSACHUSETTS, ASSIG-NOR TO GENERAL ELECTRIC C(JMPANY, A CDRPQRATIOEI OE NEW YORK.

ELECTRIGAL MEASURING INSTR-llMENT.

Specification of Letters Patent.

Fatenteti Apr. 9, 1%12.

Application filed July 30, 1908. Serial No. 446,683.

To all whom it may concern:

Be itknown that l, ELIHU THOMSON. a citizen of the United States, residing at Swampscott, county of Essex, State of Mas sachusetts, have invented certain new and useful improvements in Electrical Measuring Instruments. of which the following is a specification.

My invention relates to electrical measuring instruments having current carrying coils mounted to move in the field produced by permanent magnets, and its object is to provide a measuring instrument which has an open, legible and uniform scale; in which the torque is strong and does not tend to cause binding of the moving system at the pivots; and in which strong damping is secured without increasing the weight of the moving system.

My invention will best be understood in connection with the accompanying drawillustration of some of the various forms in which the invention may be embodied, and in which-,

Figure 1 is a plan view of a measuringinstrument; Fig. 2 a cross-sectional view of the instrument shown in 1; Fig. 3 a plan view of the instrument shown in Fig. 1 with a portion broken away to show the details of construction; Fig. 1 a view in elevation showing the annular pole pieces, the damping disk, and the equalizing flange of the instrument shown in Fig. 1; Fig. 5 a cross-sectional view or" a. i'noditication in which two current carrying coils are used; Fig. 6 a view in elevation showing the relation of the annular pole pieces and of the current carrying coils of the instrument shown in Fig. 5; and Fig. 7 a perspective view or a pole piece which may be used in either form of instrument.

The instrument shown in' Fig. 1 is circu lar in form, and comprises a heavy base 1, which is secured to the switch-board or other convenient support and carries a cover 2 formed in partof a glass plate. The base is provided with terminals 3 by means of which current is led to the moving coils of the instrument and as shown in Fig. 2 is preferably recessed to receive a base plate 4 provided with pedestals 5, which support a scale or dial 6 over which the pointer of the instrument swings.

The instrument has a magnetic system comprising a permanent magnetformed as shown in Fig. 2 and secured to the base plate 4. by clamps or other securing means. The magnet has pole pieces S'and 9 mounted parallelfo each other and separ: ted by a narrow air gap in which an intense magnetic field exists. The pole pieces. preterably annular in form, are mounted eccentrically on the magnet poles, by which I mean each of the magnet poles engages a pole piece a point near its periphery. Although the poles are out oi .alincment, the pole pieces are superimposed so as to be parallel to each other and to have a common axis and are separated by an air gap in which exists a strong uniform annular field through which the current carrying coil moves, whereby an instrum nt is Secured which has a long uniform scale, as shown in Fig. 1.

The moving system of the instrument is supported. by brackets 10 carried by the pedestals and having bearings for the arbor or pivot 1, which is mounted concentric with t e om'mon axis of the annular pole pieces of the permanent magnet 7 and carries a coil supporting member 12 preferably made in the form of a disk or" sheet metal or other suitable conducting material extending into the field between the pole pieces 8 and 9 to act as a damping disk. The current carrying coil 13 of the moving system is mounted on the damping disk 12 and as shown in Fig. 2 is arranged to encircle the lower pole piece 9 of the perma nent magnet 7 to bring a portion of the current carrying coil. into the very intense and uniform magnetic field between the pole pieces. The current is carried to the coil 13 by spiral flexible leading-in conductors 14 connected to the terminals 3 through suitable loads 15 and a resistance 16 which acts as a compensator. it. pointer 17, mounted on the arbor ll in a position to counterbalance the weight or the coil 13, swings over the long scale plate 6, which is uniformly graduated as shown in Fig. 1 and is very shown in Figs. 5, 6 and 7 is the same in generai construction as the instrument just described, butdifiers therefrom in that two current carrying coils are symmetrically mounted on the damping disk 12 in such a manner that one coil encircles one pole piece and the other coil diametrically opposlte it encircles the other pole piece and hence the forces which tend to turn the moving systein on its pivots are equalized and are applied in opposlte directions at opposite ends of a diameter of the moving system. As

shown in Figs. 5 and 6 a plurality of current carrying coils 13 and 18 are symmetrically mounted on opposite sides of the As best shown in Figs. 2 and 3, the con fronting annular pole pieces, which are in difl'erentplanes, have a common axis and are mounted parallel. to each other. Each pole piece is provided with a slot or opening 20 to permit the moving system to be placed in position, and since the slot in the lower pole piece 9 is in the path of movement of the coil'it is preferably closed after the'moving system is in place by an iron plug 21 as shown in Fig. 7 thereby making -the pole piece a continuous ring of magnetic material, while the slot in the upper pole piece 9 has no effect on the movement of the coil and may be left open. Owing to the-relation of the poles of the mag net, the magnetic flux is more dense adjacent the poles than at other points in the annular air gap unless some profluence of, the

shield or fiangeis required on the lower vision is made to. equalize the flux and 1n accordance with my lnventlon the magnetic flux is distributed uniformly throughout the path of movement of the current carrying coil 11 by means of an equalizing member or projection 22, preferably in the form .of an annular flange secured'to or forming part of the upper pole piece 8 and proportioned to produce a uniform fiux bctween the pole pieces throughout the desired distance. The flange extends without interruption under the upper pole 8 wherethe flux has a tendency to concentrate, but is provided with a gap 23 over the pole 9, because the current carrying coil does not move over that pole andthe equalizinginflange isn'ot required. No

pole piece 9 which is attached. to the magnet pole at a place which is not traversed by the current carrying coil. The flange may other forms than that pieces, and

be tapered or otherwise altered in shape produce any desired distribution /of the flux, but is preferably made annular and, of' uniform cross-section, as shownin thedraw'ing. 5

Since the damping member is madein the form of a disk some provision mnst be made to permit the current induced in the disk to return or flow back in order to.

rated by a weaker field due to the presence of the radial grooves '25.

the magnetic field due to: grooves has nothing to do with deflecting the current coil, but acts solely to produce the damping effect on the disk which, in

The portion of all positions of the moving system, extends" through the fields produced by'the horns and grooves. Since the magnetic field is practically abolished between the 'radial 5 the horns and grooves 25 the dampingcurrents"developed in the'disk by the intense fields between the opposing ends of the horns 24 can return or flow back by a short return circuit in the n utral space between the grooves, thereby producing .21 strong niagnetic field between the two annular ole piece? is therefore divided into two distinct and independent portions, one of which is proportioned to secure a strong damping while the other is traversed by. the current coil and has the flux made uniform throughout the path of the coil.

My invention may be embodied in many and I do not limit it to the precise arrangement disclosed but intend to cover by the appended claims all changes and modifications within the spirit and scope of my invention.

.VVhat I claim as new and desire to secure by Letters Patentof the United States, is,'-

1. A measuring instrument comprising a magnet having confronting parallel annular pole pieces eccentrically mounted on the poles of said magnet, one of said pole pieces having a flange adaptedto counteract irregularities in the field due to the relation of the poles of thernagnet and thereby produce damping effect. The

shown and described,

a uniform magnetic flux throughout a definite portion of the air gap between said pole a current coil mounted to move in said portion ofsaid air gap.

,2. A measuring instrument comprising a magnet and confronting parallel annular pole pieces ecoentrically mounted on the 1poles of said magnet, one of said pole pieces .aving an equalizing flange shaped to pro.-

bination with a magnet having annular pole pieces mounted side by side and parallel to each other one of said pole pieces having a flange and a radial groove adapted to produ'ce an annular field having adjacent portions of diflerent flux density, of a moving system comprising a damping member of 1 conducting material extending through both said adjacent portions ,of different flux 5 density.

5. In a measuring instrument, the combination with a magnet having annular pole pieces mounted parallel with each other and side by side and provided with radial grooves in the adjacent faces to produce a weak field between twostrong fields, of a moving system comprising a damping disk pivoted concentric with said pole pieces and extending through all said fields in all positions of the moving system.

6. In a measuring instrument, the combination with a magnet having confronting pole pieces, each provided. with projections separated by a recess, said projections cooperating to produce strong fields separated by a weak field due to saidrecesses, of a moving system comprising a damping member of sheet metal mounted to extend through all said fields in all positions of the moving system.

7. In a measuring instrument, the combination with a magnet having annular pole pieces mounted side by side and parallel to each other and one of said pole pieces having a flange and a radial groove adapted to produce an annular field having one portion of'uniform strength and a second portion comprising two strong fields separated by a weak field, of a moving system comprising a current carrying coil mounted to move in one portion of said field, and a damping member mounted to move in the second portion. v

8. In a measuring instrument, the comblnation with a moving system comprising a current coil and a'damping member of sheet metal, of a magnet having annular pole pieces mounted side by side and parallel to each other one of said pole pieces having a flange adapted to produce a uniform magnetic field throughout the path of the coil, said pole pieces having opposed radial grooves outside the path of the coil to produce a magnetic flux of low density to. cut

said damping member in all positions of the moving system.

In witness whereof, I have hereunto set my hand this twenty eighth day of July, 1908.

ELIHU THOMSON.

Witnesses:

JOHN A. MCMANUS, J12, HENRY O. ns'rnNDonr. 

